High performance coatings formulated from polyvinylidene fluoride (PVDF) polymer resins are known to provide good solvent resistance, chemical resistance, weather resistance, heat stability, strength and resilience. PVDF solvent-base coatings (e.g. KYNAR 500®) have been usually used on metal substrates. U.S. Pat. Nos. 8,093,329 and 7,399,533 describe coating compositions based on non-aqueous dispersions of solid PVDF particles in an organic solution of acrylic polymers. The coating compositions contain PVDF powders, organic solvents, acrylic polymers, and optionally pigments and colorants. After baking the coating above the PVDF melting temperature, a homogenous blend of PVDF and acrylic phase is formed which provides the coating with durability and other essential properties such as gloss, adhesion, solvent resistance, and weatherability.
PVDF-based paints commonly contain high levels of organic solvents, which are needed to dissolve acrylics and disperse PVDF. These organic solvents are typically classified as volatile organic compounds (VOCs) and are often regulated in their use. The organic-solvent-based slurry presents safety, health and environmental dangers that are not present in an aqueous system. Organic solvents are generally toxic and flammable, volatile in nature, and involve special manufacturing controls to mitigate risk and reduce environmental pollution from the organic solvent. In addition, a large carbon footprint is associated with use of organic solvents that is not environmentally desirable. Further, extra manufacturing steps, costing time, money, and energy are involved to isolate PVDF formed in an aqueous media, drying the PVDF to a powder, and then dissolving/dispersing the powder in a solvent.
There is a need for a fluoropolymer-base coatings and films which maintain these excellent properties, but which are more environmentally-friendly and aqueous-based.
To effectively employ waterborne slurries in coating formulation processes, it is important to develop binder systems that are compatible with current manufacturing practices and provide desired properties of the intermediate and final products. Some common criteria include: a) stability of the waterborne fluoropolymer dispersion, having sufficient shelf-life, b) compatibility with acrylic latexes c) stability of the slurry after admixing the powdery materials and pigments, and d) appropriate viscosity of the slurry to facilitate good aqueous casting. Additionally, from a regulatory view, fluoropolymers made without fluorosurfactants are preferred.
PVDF emulsions generally provide particle diameters above about 250 nm. These emulsions have reduced colloidal stability and are highly unstable when neutralized, or used under high shear conditions such as in a spray application.
One means to provide a stable, aqueous PVDF composition is through fluoropolymer and acrylic hybrid compositions made by a sequential emulsion polymerization process, as taught in U.S. Pat. No. 5,804,650 and US2011/0118403. These hybrid compositions involve the polymerization of acrylic monomer(s) in the presence of a fluoropolymer seed resin, producing the PVDF/acrylic hybrid emulsion. This aqueous hybrid composition can not be used at high baking condition due to the phase separation at elevated temperature (e.g. beyond 150° C.). The coating based on this hybrid latex requires a complicated polymerization processes while exhibiting inferior chemical resistance and hot dirt pick up resistance compare to this disclosure.
U.S. Pat. No. 3,301,807 describes an aqueous dispersion of PTFE fluoropolymers containing non-ionic surfactants where the fluoropolymer was made by using per-fluorinated surfactants. U.S. Pat. No. 6,696,512 describes fluoropolymer, and especially PTFE, aqueous dispersions made with fluorosurfactants, and having particle sizes of 240 microns and larger.
Stable fluorosurfactant-free PVDF emulsions have been produced by Applicant, using a variety of different surfactants, and producing emulsions having a particle size in the range of from 50 to 250 nm. Useful surfactants include 3-allyloxy-2-hydroxy-1-propane sulfonic acid (U.S. Pat. No. 6,869,997); polyvinylphosphonic acid, polyacrylic acid, polyvinyl sulfonic acid (U.S. Pat. No. 8,869,997), polyethylene glycol, polypropylene glycol, polytetramethylene glycol block co- and ter-polymers. Aqueous-based dispersions have been combined with powdery electrode forming materials and applied to electroconductive materials to form electrodes, as described in US 2010-0304270.
Surprisingly, a high shear stable aqueous fluoropolymer coating composition has been found, that is useful in high-bake coating applications. The fluoropolymer coating combines a fluoropolymer and a specific acrylic polymer to form a composition that is stable at high-bake temperatures of 200° C., and is shear stable under high shear forces, such as in a spray application. The aqueous fluoropolymer coating is useful in many applications, including protective and decorative coatings on metals and ceramics, as well as for the coating or impregnation of textiles, glass fibers, aramid fibers and carbon fibers.
The aqueous coating composition of the invention provides many advantages over similar solvent-based fluoropolymer coatings, including: it is more environmentally friendly and safer—with little or no exposure to solvents or solvent waste; it is easier to process (faster and cheaper) than similar solvent-based coatings—since the PVDF is synthesized as an emulsion, no separation step is required; the aqueous coating can be dried at a lower temperature than the solvent coatings—saving time and cost; higher gloss with a 60° gloss of greater than 60, and preferably greater than 70. It is believed that the higher gloss is related to the polymer particle size in the dispersion being less than in the solvent-based coating compositions.
Further, dry coatings of the coating composition of the invention have excellent chemical resistance (MEK rub), and excellent dirt pick-up resistance.